The present invention relates to the field of batteries and, more particularly, to button cells and cylindrical batteries.
A problem common to button cells and cylindrical batteries is electrolyte leakage prior to the normal end of their useful life. In other words, the useful life of these batteries is often prematurely terminated due to electrolyte leakage.
The consequences of electrolyte leakage are two-fold. The first is that, since the batteries are not used to their full working life, the net cost of the battery increases. The second is that electrolyte leakage can damage expensive equipment.
While it would be desirable to eliminate electrolyte leakage from batteries, the problem of electrolyte leakage remains largely unsolved.
The battteries with which this invention is most concerned are sealed in two ways. One type of seal, commonly used for alkaline batteries, is a crimp seal. In a crimp seal, a flange on the outer part of the battery can is crimped over an edge of the cap. Sealing material is interposed between the flange and the cap so as to seal the battery can and also to help prevent electrolyte leakage.
After the crimping force is removed, there is a certain amount of springback of the flange material. On a stress/strain curve, this springback would coincide with the elastic strain portion which is removed upon removal of the stress. Once springback occurs, pressure on the sealing material is relieved to a certain extent. This allows the phenomenon of electrolyte creep to begin. In this regard, see "Electrolyte Creep in Galvanic Cells/ I. Contribution to the Phenomena", H. W. Nientiedt, Journal of Power Sources, 8(1982) 257-265. Eventually there is electrolyte leakage.
The second type of seal is a glass-to-metal or ceramic-to-metal seal. In this seal, an opening in the cap is filled by glass or ceramic and an electrode. The glass or ceramic, besides holding the electrode in place, serves also to insulate the electrode from the remainder of the can.
The glass-to-metal or ceramic-to-metal seal is frequently used in conjunction with lithium batteries but may also be used with alkaline batteries. In either situation the glass or ceramic suffers from chemical attack which leads to degradation of the seal and subsequent failure of the battery. Failure of the battery here may be by electrolyte leakage, as with the crimp seal, or it may be by the formation of a conductive layer on the glass or ceramic leading to a shorting out of the battery. In this regard, see "Glass/Ceramic/Metal Seals for Battery Applications", Topping et al , Journal of the Canadian Ceramic Society, 45(1976) 1-4; and "Glass-to-Metal Seal Corrosion in Lithium-Sulphue Dioxide Cells", Bunker et al., Power Sources, 8(1981) 53-62.
It is apparent that an improved battery seal would be desirable.
The present invention proposes the use of a heat recoverable, or more preferably a shape memory alloy, element as an essential part of the battery casing. It is believed that the proposed use of the heat recoverable element or the preferred shape memory alloy element will result in an improved battery seal.
Materials, both organic and metallic, capable of being rendered heat recoverable are well known. An article made of such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The article is said to be heat recoverable for the reason that, upon the application of heat alone, it can be caused to revert, or to attempt to revert, from its heat-unstable configuration to its original, heat-stable configuration.
Among metallic alloys, the ability to possess shape memory is a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change in temperature. This tranformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such an alloy, for example a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as M.sub.s and the temperature at which it finishes M.sub.f. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as A.sub.s (A.sub.f being the temperature at which the reversion is complete) the deformed object will begin to return to its original configuration.
Various proposals have been made to employ shape memory alloys but, it is believed, none have encompassed the use of shape memory alloys to seal a battery casing.
In view of the above, it is an object of this invention to have a battery casing with an improved seal.
Another object of the invention is to have a battery casing with an improved seal that employs a heat recoverable element.
A further object of the invention is to have a battery casing with an improved seal that is simple in design and economical to produce.
These and other objects of the invention will become apparent after reference to the following description considered in conjunction with the accompanying figures.